Model chart: an example starting at 35 psi and 10°C becomes 30.6 psi at minus 15°C. The pressure axis begins at 30 psi.
Illustrative model: fixed air amount, fixed volume, standard atmospheric pressure and temperature equilibrium. The vertical axis begins at 30 psi. This is not a measurement or an inflation target.

Rested is not the same as outdoor-cooled.

A tire can sit overnight in a warm garage and still contain warmer air than the air outside. Transport Canada explains that a falling temperature lowers tire pressure. The same quantity of air can therefore produce a different gauge reading after the tire cools.

That matters when you compare an indoor reading with an outdoor reading. Neither resting for three hours nor entering the outdoor weather into a calculator proves the tire has reached that temperature.

Try the problem, with the assumptions visible.

Illustrative conditions: a tire set to 35 psi at 10°C, then allowed to reach −15°C. With fixed volume, no air loss and standard atmospheric pressure, the model estimates 30.6 psi. The starting 35 psi is an example, not a recommendation for your vehicle.

This is a model result derived from the ideal-gas relationship. It does not predict cooling time, diagnose a leak, or tell you how high to inflate a warm tire.

Open this example in the calculator

Build a check plan.

  1. Find each tire’s specification. Front, rear and spare requirements can differ. Note any load-specific instructions in the manual.
  2. Record conditions with the number. Write down garage or outdoor location, ambient temperature and whether you drove recently.
  3. Choose a rested recheck. Where practical, check after the vehicle has rested in the conditions you intend to assess. Follow the manufacturer’s procedure.
  4. If reaching the pump requires driving, preserve that context. The reading at arrival is warm. Use the manual or qualified assistance for inflation guidance and arrange a subsequent cold check.

Do not solve uncertainty with an invented target.

A rule such as “always add a few extra psi for a warm garage” ignores the vehicle, actual tire temperature and air loss. This site does not produce a universal warm-fill target.

If a tire is seriously low, damaged or repeatedly loses pressure, stop relying on the temperature estimate and seek appropriate help. NHTSA treats warm inflation when a tire is believed low as a temporary situation requiring a later cold recheck; it is not a reason to assume a trip to the pump is safe.

Record a garage reading and its conditions so your next comparison has context.

Sources & scope

  • Transport Canada

    Individual front, rear and spare specifications; temperature-related pressure changes. Page modified December 10, 2013.

  • NHTSA TireWise

    U.S. guidance: cold readings, vehicle placards, monthly checks and TPMS warning patterns.

  • NASA Glenn: ideal-gas equation

    Ideal-gas relationship and absolute temperature; supports the model, not tire-specific validation.

Primary sources checked November 5, 2026. This is not a vehicle-specific inspection.